I have to go with Call of Duty on this one. I haven't played much of any Halo game outside some brief minutes with the original game and 3 or so missions of the Reach Singleplayer Campaign. Just from what I've seen, it would be difficult to imagine an FPS more generic, bland, and archaic than the Halo series. It feels like a 3D re-skin of the original Doom, with all the gameplay complexity that entails. I can at least find enjoyment in the Call of Duty Singleplayer campaigns, as well as increasingly decent storytelling.